The present invention relates generally to a novel method of processing a heated workpiece utilizing a fluidized bed, which acts as both a cooling media and a universal fixture.
In superplastic forming, a process finding increased acceptance in the aircraft industry, a sheet metal blank having superplastic characteristics is formed to complex shapes with precise tolerance at elevated temperatures (in the range of 1500.degree.-1750.degree. F. for titanium alloys) and under pressure conditions, where the blank exhibits superplastic properties. The metals used are preferably titanium, aluminum, and the alloys of each. When the blank has completely formed, the part must be cooled in such a uniform manner so as to maintain tolerances and avoid distortion. (See U.S. Pat. No. 4,233,831.) This cannot be accomplished with conventional quenching media, such as water, brine, or a salt bath.
Diffusion bonding is a process where similar metallic parts are pressed together at elevated temperatures and pressures causing deformation which results in initimate contact of the surfaces to be joined and subsequent diffusion of the atomic structure, thereby forming a monolithic metallic piece with joint strength equivalent to that of the parent metal. The metals used in diffusion bonding are titanium alloys which are susceptable of superplastic forming. For certain applications diffusion bonding can be used in conjunction with superplastic forming, or the two forming processes can be used independently of each other since both processes occur at elevated temperatures. These structures must be cooled from these elevated temperatures without warpage. The most common alloy used in superplastic forming/diffusion bonding is Ti-6A1-4V.
Normally structures fabricated from titanium alloy sheet are not heat treated to higher than recrystallized annealed temperatures, since the severe quench cooling rate required creates a severe distortion problem. Similarly, products formed from aluminum alloy sheets require fixturing to survive the quench rates imposed during strengthening heat treatments. Fixture tooling can be expensive and generally will be specific to a given configuration. For titanium alloy sheet structure the high temperatures involved preclude the use of water quenching.
Fluidization of particulate, solid matter is well known, and is currently used in many process industries. Conventionally, a fluid under pressure is passed through a porous diffuser and introduced into a bed of finely divided solid, particulate material. The flow rate of the pressurized fluid is sufficient to levitate and agitate the solid particles thereby imparting fluid characteristics to the bed.
A high rate of heat transfer is possible when a workpiece is immersed in the fluidized bed and there is a substantial temperature differential between the workpiece and the bed. This is caused by the turbulent motion, rapid circulation rate of the particles, and the large amount of surface area per unit volume of the solid particulate material. Even though the heat transfer coefficients for a particulate material are not usually high, the amount of surface area per unit volume is large: for ordinary sand, the surface area to bulk range is from 1000 to 5000 ft.sup.2 /ft.sup.3. The heat transfer coefficient of a fluidized bed is usually between 20 and 210 Btu/ft.sup.2 .multidot.hr.multidot..degree.F., which is comparable to salt or lead bath equipment. The primary advantage of the fluidized bed approach is that the process remains essentially isothermal. Other advantages include an easily varied contact time, and an apparatus that can be reused and is readily adaptable to continuous, automatic operations.
A new cooling method is required so that heated workpieces of complex shapes involving sheet metal fabrication may be cooled at a uniform and controlled rate, so that metal strength properties can be optimized while minimizing distortion.